There is a demand, particularly in remote areas, to provide underpass systems which include overpasses and which can carry not only dead loads, but as well live loads. Such installations may be associated with mining or forestry industries, where vehicles of substantial tonnage pass over or pass under the structural systems. There is also a continuing demand for overpass and underpass structures for highways and other types of roadways where the installation has the usual life expediency and is cost-effective. Other needs for overpasses are in respect of constructing bridges and the like where there is minimal disturbance to the river bed. Such overpasses may also have restrictions in terms of height of the overpass and slope of approach, which restricts to some extent the design of the overpass. Although, many of these demands can be met with concrete structures, they are very expensive to install, are cost prohibitive in remote areas and are subjected to strength weakening due to corrosion of the reinforcing metal and hence, repair.
There have been significant advances in respect of the use of corrugated metal culverts, arch culverts and box culverts, such as described in U.S. Pat. No. 5,118,218 which use sheets of metal having exceptionally deep corrugations where by, using significant material on the crown portions of the culvert and perhaps as well in the haunch portions of the culvert, significant loads can be carried by the culvert design. Ovoid and circular structures are described for example, in U.K. patent application 2,140,848 where wing members are used to increase the load carrying capabilities, and in particular avoid bending of the crown or roof structure as live loads pass thereover.
Applicant has described in U.S. Pat. No. 5,326,191 a reinforced metal box culvert which is provided with a special form of continuous reinforcement along at least the crown or top portion of the culvert. Significant advantages are provided in load carrying characteristics, reduced overburden requirements and the ability to provide large span structures that reduce the cost. Improvements to the box culvert and arch culvert designs are also described in applicants U.S. Pat. No. 5,375,943 and International application PCT/CA97/00407. These systems greatly facilitate the installation of large span structures with the ability to carry live loads under a variety of conditions.
As the installation of corrugated metal culvert structures gain acceptance, there is a greater demand for these structures to accommodate very large spans usually in excess of 6 meters and its well extended sidewall height usually also in excess of 6 meters. Although, these structures can be made to structurally resist both dead and live loads after installation is complete, backfilling of the structure presents, a significant problem, because of the deformation of the crown of the arch structure and/or extended sidewalls of the box culvert structure.
The use of reinforced earth in archway construction is described in U.S. Pat. No. 4,618,283. Such construction technique avoids arching of the structure because the sidewalls of the archway are built as successive layers of reinforced earth which are deposited along side and over top of the structure. The technique involves building on each side of the archway reinforced earth which constitutes vertical support sections, and then building across the top of the arch again using reinforced earth to define the roof of the archway. As the archway is built step-by-step, facings are applied to contain the reinforced earth and prevent such compacted unbound fill of the reinforced earth structure from coming loose and falling into the archway. Such mat faces may be simply attached to the vertical portions of the wire mesh which terminate at the edge of the archway envelope. Alternatives to the facing material include spraying of concrete to provide a liner within the archway or the use of a corrugated metal liner. Optionally, the reinforcing mats of the reinforced earth vertical structures may be attached to the corrugated metal liner. The liner is not designed to carry any structural load either live or dead, instead the live and dead loads are carried by the reinforced earth vertical support sections as well as the reinforced earth roof section.
The use of reinforced earth is also discussed in Abdel-Sayed et al., "Soil-Steel Bridges" McGraw-Hill, Inc- chapter 8, page 269. The use of soil reinforcement by strips of steel attached to the sides of a horizontal ellipse pipe structure are described. The apparent benefit of the use of these steel strips include greater load carrying capacity for the pipe, by reducing axial thrust and almost eliminate bending moments due to live load in the conduit wall and among other things restrain the movement of the pipe during the backfilling operation. However, the authors of that book sincerely doubt the benefit of connecting the steel strips to the pipe, because it would restrain movement of the pipe during backfilling and prevent the development of full soil support to the pipe and as well create the hard point effect at all locations where the pipe is connected to the steel strips. It is generally understood by those skilled in the art when backfilling pipe structures that it is important to allow the side segments of the pipe to mobilize so that the maximum support of the soil can be achieved in carrying live and dead loads. The authors however, do believe that the use of steel strips above the pipe is beneficial and is indeed similar to the structure advocated in U.S. Pat. No. 4,618,283 where a reinforced earth is provided above the archway as well as on the sides.
It is well known that the thrust in a soil-metal structure is the product of the radius of the structure times the soil pressure surrounding the structure. In a typical installation, an active earth pressure is exerted on the sidewalls of the structure during backfilling. This active pressure pushes the sidewalls in and the crown or top wall up. As the backfilling progresses over the crown, an active pressure is applied to the top of the structure pushing the crown down and the sidewall out. The pressure on the sidewall then changes from active to passive. It is obvious, in this relationship, that since the thrust is fairly constant, small radius structures will produce large pressures and large radius structures will produce small pressures. The concerns of Abdel-Sayed relate to a horizontal ellipse structure in which the radius of the sidewall is much less than the radius of the crown. In a horizontal ellipse, circular pipe, pipe- arch or plain arch, the sidewall is encouraged to move inward during backfilling in order to develop more passive pressure, when the crown is backfilled and the sidewall pushes out. H. Mohammed et al "Economical Design for Long-Span Soil-Metal Structures" Canadian Journal of Civil Engineering, vol. 23, 1996, pages 838-849 describe the use of reinforced soil with horizontal ellipse culvert having a larger radius crown and a small radius sidewall. The reinforcement of the reinforced soil is attached only to the upper sidewall of the horizontal ellipse culvert and reinforced soil to a depth of 2 meters is provided above the culvert. This system is designed for withstanding live and dead loads on the structure, but does not in any way address the problems associated with backfilling because with horizontal ellipse structures, backfilling is not a significant problem.
In a re-entrant arch type culvert or a box type culvert with an extended sidewall, the situation is substantially different. In a re-entrant arch type culvert the radius of the sidewall is quite large compared to the radius of the crown. The passive pressure required to stabilize the sidewall is much less than in a horizontal ellipse culvert.
In a box culvert, with an extended sidewall, the radius of the sidewall is infinite since the wall is straight. There is no passive pressure on the sidewall pushing it out. Instead the sidewall must resist active pressure from backfill which pushes in.
Quite surprisingly, in accordance with this invention the use of reinforced earth wherein the reinforcement is attached to the side portions of the culvert or underpass during backfilling provide a significant benefit in minimizing or preventing deformation of the crown and sidewall of the culvert or underpass.